The invention concerns a new fused cast refractory material formed of .beta. alumina which is particularly suitable for constructing glassmaking furnace superstructures, among other applications.
Fused cast .beta. alumina refractory materials have been known for many years. U.S. Pat. No. 2,043,029 describes materials containing alumina and 1-10% sodium oxide and indicates that approximately 5% Na.sub.2 O is sufficient to obtain a material essentially formed of .beta. alumina and that the presence of more than 1% of silica and titanium dioxide must be avoided because the latter impede the formation of .beta. alumina. U.S. Pat. No. 2,043,029 does not specifically describe materials containing more than 5% Na.sub.2 O and does not suggest that such materials can have a particular advantage.
SU-A-391 103 proposes the addition of sodium in the form of aluminate NaAlO.sub.2 (1% to 15%) to prevent volatilisation of the sodium substances. This Russian patent does not specifically describe any product containing more than 7.04% Na.sub.2 O and does not give any information as to the effect of the composition on the characteristics of the product.
According to patent FR-A-2739617 compression resistance can be improved by adding BaO, SrO and CaO to products with a total NaO+K.sub.2 O content varying in the range 4% to 7%.
In practice, all commercially available .beta. alumina materials, such as MONOFRAX H from Monofrax Company (U.S.A.) or Toshiba Refractories (Japan), MARSNIT.RTM. from Asahi (Japan) or JARGAL.RTM. H from the applicant have very similar analyses, namely 93% to 94.6% Al.sub.2 O.sub.3, 5.2% to 7% Na.sub.2 O and 0.1% to 0.3% silica and other oxides (impurities or intentional additions).
Other .beta. alumina products are commercially available in which the amount of sodium oxide is as much as 6.7% but the content of silicon dioxide is less than 0.05%. A product sold by the applicant under reference ER.5312, known as .beta.'" alumina, comprises 86.5% to 87.5% by weight aluminium oxide, 4.5% sodium oxide and 8% magnesium oxide, together with small amounts of other oxides, in particular around 0.30% silicon dioxide. All the above products contain virtually no corundum or a alumina (less than 34% and typically 2% maximum in JARGAL H).
.beta. alumina materials have a high resistance to thermal shock and to mechanical stresses at high temperature. These properties make them suitable for use in superstructures in the melting chamber of glassmaking furnaces. However, .beta. alumina materials are somewhat fragile. Accordingly, machining and cutting cast blocks are difficult and costly because of wastage. Also, the various handling operations that the blocks undergo during manufacture and during assembly of the furnace can cause nicks in edges and damage corners. Even a highly localised absence of material in a superstructure block can lead to the risk of accelerated corrosion of the material associated with the presence of deposits of corrosive materials emanating from the glass or simply from an increase in the corrosion surface area of the block.
Moreover, damage to the blocks during manufacture increases the cost of the product by reducing the yield. This problem limits design solutions because superstructures generally require parts with very clearly defined edges to a greater extent than in other applications.
Moreover, the trend in glass melting techniques is towards more severe conditions in which the materials of the furnace are more stressed and this applies in particular to the development of oxy-gas combustion.
Because of the requirement for integrity of structures, more pressing than in the past, in particular at the level of the joints, and to contribute to the current evolution in superstructure geometry, there is a need for fused cast products consisting principally of .beta. alumina having improved mechanical properties.